Tachometer for Mercury Mark 55

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Tachometer for Mercury Mark 55

Post by tottenisland »

I have a 1955 Mk 55 40hp motor on the back of a Tuffy, and I'd like to put a tach on it. I'm in the process of tuning the motor and trying to get the right prop on it. Currently, I've been getting by with a small digital tachometer meant for chainsaws that gets its reading via induction from a wire wrapped around a spark plug lead. This works fine, with a bit of shielding, but I'm looking for something more permanent/elegant. Ideally I would be able to have an analog dial mounted on the dashboard.
I've done some digging on these forums and on the greater internet, but haven't found much info on hooking up a tach to a motor of this age. I've seen a bunch of really nice builds using this motor so I'm willing to bet someone has figured this out. Does anyone reading this have any experience in this area?


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Re: Tachometer for Mercury Mark 55

Post by gdcarpenter »

The $64,000.00 dollar question is do you have a charging system on your Mark55. If you do no problem. Since these are magneto ignition engines if you have a charging system you can get a marine tach that 'reads' the impulses from the charging system stator. The tach signal feed needs to be connected to one of the yellow leads coming from the stator and connected at the rectifier, and the switch on the back of the tach switched appropriately.

Whatever the total number of coils on your stator, divide by 2 and that is your tach setting, because half of the stator coils feed each of the two rectifier inputs. I believe the 44 cubers have 12 pole stators, ergo the tach would need to be switched to '6'. Ran such tachs on my Mark35A and Mark58. If in doubt check the experts at 'Johns Old Mercury' site.

You should be looking at about 5,000-5,600 RPM at WOT.
This is my first, last and only boat build.


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